This is not allowed because you can do more than this with abstract classes. It wouldn't make sense to allow multiple inheritance, provided you only used an abstract class when you could have used an interface.

It is simpler to only use abstract classes for things you can't do with an interface, in which case you wouldn't be able to use two abstract parent classes.

Note: with Java 8 there is less you can't do with an interface, you can have public instance and static methods with implementations.